Function saturn_ring_crossing 

pub fn saturn_ring_crossing(
    spacecraft_pos_km: Vec3<f64>,
    approach_vel: Vec3<f64>,
    jd: f64,
) -> RingCrossingAnalysis

Analyze whether a trajectory approaching Saturn crosses its ring plane.

Given a spacecraft position relative to Saturn and approach velocity vector, determine if and where the trajectory intersects the ring plane.

spacecraft_pos_km: position relative to Saturn in ecliptic frame (km) approach_vel: velocity vector in ecliptic frame (unit direction) jd: Julian Date (for computing ring plane orientation)